The production of copper foil for electronic application, i.e., copper-clad laminates for printed circuit boards, consists essentially of two steps; first, electrodeposition or plating, of a "base" or "raw" foil on a rotating drum-cathode and, second, passing the "base" foil through a "treater" machine, in order to provide the matte side of the foil with a bondable surface suitable for bonding to a polymeric substrate. The latter step is sometimes called the bonding treatment. Traditionally, these two operations are separated by the foil manufacturers, since they seem to be mutually exclusive; formation of base foil calls for a concentrated, hot copper sulfate/sulfuric acid electrolyte, in order to yield strong, ductile and compact deposits which form the body of the foil, while the bonding treatment usually requires a more dilute and colder electrolyte which yields fragile, powdery deposits whose role is to enhance the true surface area of the matte side of the foil and thus enhance the bonding ability of the foil.
In a typical process, the first step, fabrication of the base foil, or "core," a primary objective is to impart to the bulk of the foil the combination of physical, metallurgical and electrical properties desired in the printed circuit industry, and obviously, those properties are determined by the microstructure, which in turn is determined by purity and conditions of the plating process. Typical properties of the core of the foil sought by printed circuit board manufacturers are suitable tensile strength, yield strength, elongation, ductility and resistance to fatigue. Many of the desired properties relate to the maximum load the material may withstand before failure, and are usually derived from stress-strain curves. Similarly, electrical conductivity is considered an important property of copper foil. All these properties of copper foil depend on the foil's microstructure, but particularly on the microstructure of the core of the foil. This microstructure, responsible for foil's properties, is in turn determined by the electrodeposition conditions.
Similar to other materials used in high technology applications, copper foil is a composite; i.e., it has a near-surface region with properties differing from those of the bulk material. Thus, the bulk of the copper foil (core) serves in printed circuit boards as the conductor of electricity. The matte side of the foil is responsible for promoting a permanent bond to the polymeric dielectric (insulating) substrate or prepreg, e.g., glass fabric impregnated with epoxy resin.
As shown in FIG. 1, the first manufacturing step utilizes a large cylindrical drum-cathode 20 which rotates, partially immersed in a copper sulfate-sulfuric acid electrolyte 21. The drum cathode is adjacent to and facing toward a pair of curved anodes 22, which may be formed of lead, lead-antimony, platinized titanium, iridium or ruthenium oxides. Both the drum and the anodes are connected electrically by heavy buss-bars 23 to a D.C. power source 24, and currents of up to 50,000 amps or more are commonly used. As the drum rotates in the electrolyte, an electrodeposit of copper forms on the drum surface, and as the latter leaves the electrolyte, the electrodeposited copper is continuously stripped from the rotating drum in the form of thin foil 25, which is slit to size and wrapped around a take-up roll 26. The top or outer surface of the drum is usually formed of titanium.
Foil produced in such a process, prior to being treated, is usually referred to as raw foil. The raw foil is pale pink in color and has two distinctly different looking sides--a "shiny side", the side which was plated onto the drum surface and then stripped is quite smooth while the other side, the side which was facing toward the electrolyte and the anodes, is referred to as the "matte" side, since it has a velvety finish, due to the difference in the growth rate of differing crystal faces during electrodeposition of the "base" foil. The matte side surface, at this stage has a very fine scale micro-roughness and a very specific micro-topography, as shown in FIG. 2. Viewed under high magnification of a scanning electron microscope, it is composed of peaks and valleys. The peaks are closely packed cones or pyramids. The cones' height, slant, packing and shape depend, as is well known, upon closely controlled independent variables of foil thickness, current density, electrolyte solution composition and temperature and the type and concentration of the addition agents in the electrolyte and the like.
In turn, the surface quality (profile) of the matte side of the base foil determines its suitability for the application as a cladding for the laminates destined for fine line circuitry and multi-layer printed circuit boards. The criteria of suitability depends upon the quantitative evaluation of the matte side's surface roughness. A variation which gives useful information about the surface is called "Rz," which is the average deviation from the mean line of the five highest peaks and the five lowest valleys within the roughness sampling length.
The matte side of the base foil provides the basic shape of the foil surface for embedding in the resin of a substrate to promote adhesion in the copper clad laminates used in the manufacture of printed circuit boards (PCB's).
While the matte side of the foil has a certain micro-roughness it is not nearly good enough to satisfy industry need for foil's bondability. This is why copper foil manufacturers have a second manufacturing step in which a surface bonding treatment is applied to the matte side of the base foil. The term "bonding treatment" is universally used to describe the changing of the morphology of the matte side of the base foil to make it suitable for bonding to laminate resins.
The bonding treatment operation is conducted in machines called "treaters" wherein rolls of raw foil are unrolled in a continuous manner and fed into the treater by means of driven rollers (similar to the way in which a web of paper is handled in a printing machine), rendered cathodic by means of contact rollers and passed in a serpentine fashion through plurality of plating tanks, facing, in each tank, a rectangular anode. Each tank has its own supply of appropriate electrolyte and its D.C power source. Between the tanks the foil is thoroughly rinsed on both sides. The purpose of this operation is to electrodeposit on at least one side of the foil, usually the matte side, microprojections of complex shape which ensure that the foil will be firmly anchored to the base polymeric materials used in fabricating the copper clad laminates.
High peel strength (the force necessary to pull apart the copper foil and the supporting insulating substrate material) is a characteristic of the highest importance, since the mechanical support of the circuit elements, as well as the current carrying capability of PCB's, is provided by the copper foil--polymer joint. It is essential that the foil is bonded very tightly and securely to the substrate and also that such an adhesive joint can withstand all the manufacturing steps in PCB's fabrication without the decrease of the initial adhesion, which, moreover should remain constant throughout the service life of the PCB.
This bonding operation is carried out in laminating plants and involves heating and cooling cycles. Sheets of copper foil are laid upon sheets of "prepreg" (e.g., glass fabric impregnated with epoxy resin). Both materials are placed in a hydraulic press having heated pressing plates, and the two materials are pressed together under high pressure. At elevated temperatures in the resin liquefies and is forced, by the pressure, to flow into the micro-irregularities of the foil surface. This is followed with a second cycle, when both materials are cooled, while the pressure is being maintained, the resin solidifies in the irregularities of the foil surface, and both materials are firmly bonded together and very difficult to pull apart. It is the responsibility of the matte side of the foil to ensure high peel strength.
The matte side of the finished foil, i.e., the base foil plus treatment, refers to the combined effect of the micro-topography of the matte surface of the base foil (electrodeposited at the drum machine) and the bonding treatment plated upon that surface at the treater machine. Both are equally important.
The preferred bonding treatment is effected by subjecting the matte side of the base or "raw" foil to four consecutive electrodeposition steps. The first consists of the deposition of a microdendritic copper layer which enhances, to a very large degree, the real surface area of the matte side, and thus enhances the foil's bonding ability. It is followed by electrodeposition of an encapsulating, or gilding, layer the function of which is to mechanically reinforce the dendritic layer, and thus render it immune to the lateral shear forces of liquid resins in the laminating stage of the PCB's fabrication. Then, a so-called barrier layer is deposited on the dual-layer copper treatment, after which a stainproofing layer is applied.
The purpose of the dendritic deposit is to increase the "true" surface area of the matte side since that property is ultimately responsible for the bonding characteristics of the foil. The shape, height, mechanical strength and the number of dendritic microprojections per surface area which constitute dendritic deposit are the factors instrumental in achieving adequate bond strength of the foil, after all stages of the treatment are completed. The dendritic deposit, the first stage of the treatment, is relatively weak mechanically and given to unacceptable treatment transfer characteristics.
The encapsulating step of the treatment is very important, since it eliminates the foil's tendency toward "treatment transfer" and the resulting "laminate staining" which can cause the decrease of the laminate's dielectric properties. The role of this second treatment stage, is to mechanically reinforce the fragile dendritic layer, by overplating it with a thin layer of sound and strong metallic copper, which locks the dendrites to the base foil. Such a dendrites-encapsulation composite structure is characterized by high bond strength and the absence of treatment transfer. The treating parameters which assure just that are relatively narrow. If the amount of the gilding deposit is too low, the foil will be given to treatment transfer. If, on the other hand, the gilding layer is too thick, a partial loss of peel strength may be expected. These first two layers of the treatment are composed of pure copper, in the form of microscopic, spherical micro-projections.
The dual-layer copper bonding treatment may have electrodeposited thereon a very thin layer of zinc or zinc alloy, a so-called barrier layer. During the fabrication of copper clad laminates destined for PCB's, the zinc-containing layer alloys with the underlying all-copper bonding treatment by the process of heat-accelerated diffusion of metals in the solid state. As a result, a layer of chemically stable alpha brass is thus formed over the surface of the all-copper treatment. Its purpose is to prevent direct copper-epoxy resin contact, and this is why the zinc-containing layer (which during lamination is converted to alpha brass), is referred to as a barrier layer. If the bonding treatment were composed of copper only and subjected to lamination with epoxy resin systems, it tends to react with amino groups of the resin, at the high laminating temperatures. This, in turn, may create moisture at the foil-resin interface, causing the harmful effect of "measling", and possibly delamination. The barrier layer plated over the all-copper bonding treatment prevents these harmful effects.
All three stages of the treatment mentioned above, as is well-known in the art, are effected by means of electrodeposition for the purpose of changing the geometry and morphology of the matte side of the foil and assure the mechanical strength of the surface region. (U.S. Pat. No. 3,857,681, Yates et al.)
The foil treated as described above may then be subjected to an electrochemical stainproofing which changes the surface chemistry. As a result of this step, the bonding surface is rendered chemically stable. This stainproofing operation removes weak surface films, which can greatly decrease the adhesion of the foil to the substrate, and provides a stable film of controlled thickness, responsible for imparting on the treated surface "durability" of its properties.
FIG. 9 illustrates a typical bonding treatment (deposited over the matte side of the base foil shown on FIG. 2) and shows clearly that the disideratum of high bondability of the foil is achieved through the increased micro-roughness of the foil and the complex shape of the particles of the bonding treatment.
Typically, the Rz of e.g. 280 micro inches of the base foil increases to the value of e.g. 420 microinches after the application of the bonding treatment, because the micro-peaks of the foil offer a favorable geometry for the rapid supply of the ions, by diffusion, in the course of the treating process. This effect is illustrated in FIGS. 3(A) and 3(B).
Since the bonding treatment is plated over the matte side of the base foil, with its micro-profile of peaks and valleys, the treatment is plated preferentially over the peaks. The diffusion layer in the electrolytes employed in the treatment process cannot follow the contour (profile) of peaks and valleys. Thus, mass transfer to the peaks is easier and the current density distribution follows suit according to the rules of mass transfer.
Peaks are thus "overcrowded" with the bonding treatment at the expense of the valleys. This is an undesirable condition of so-called poor micro-throwing power.
Treatment with overly high micro-projections concentrated on the peaks of base foil is a poor raw material for fabrication of printed circuits. The cross-section of the foil is chain-saw like, with "teeth" that penetrate very deeply into polymeric substrates. Consequently, it increases the time necessary to etch away unwanted copper, the particles of copper tend to remain deeply embedded in the resin, affecting unfavorably dielectric properties of printed circuit boards and diminishing layer to layer dielectric thickness in the fabrication of multi-layer printed circuit boards. Obviously, a reverse of described condition, a good micro-throwing power, is desirable, in the electrodeposition of bonding treatment. It is achieved, if the treatment's microprojections, instead of overcrowding micropeaks, descend deeper toward the micro-valleys.
Good bonding ability is achieved not by excessive height of the treatment at the peak, but by better distribution of the individual treatment particles (microprojections). If the height of microprojections is decreased, but their number increases, the bonding ability of the foil will remain the same, but foil will be endowed with more desirable characteristics, namely low profile.
Metallographic cross-sectioning of the copper foil clearly illustrates the reference to the foil's profile. Obviously, a foil's two opposing surfaces are not the same. While the surface formed next to the drum, the shiny side of the foil, even when viewed under great magnification, is relatively flat and smooth, the matte side of the foil comprises the micro-peaks and micro-valleys of the base foil with the bonding treatment, in the form of spherical microprojections, plated over micro-peaks and micro-valleys and concentrated especially over the micro-peaks. Thus, it can be clearly seen that copper foil has a "core" (a solid body of dense metal) and a "tooth", a chain-saw like dense coating of microprojections composed of micro-peaks of base foil and the bonding treatment. According to this convention, typical copper foils offered by the manufacturers of electrodeposited copper foil can be classified as (a) very low profile (Rz of bonding surface of 1 ounce foil e.g., combined roughness of the matte side of the base foil and the bonding treatment must not exceed 200 microinches), (b) low profile foils, when Rz should not exceed 300 microinches, and (c) so called "regular" foils, when the foil's profile can be higher.
Only the first two categories are generally used for the fabrication of multilayer boards. Naturally, that raises the question how to determine the gauge of copper foil destined for electronic application--the weight per unit of surface area versus the actual thickness. The former is most often used, and foil weighing one ounce per one square foot is called one ounce foil (1 oz.).
Such designation is now considered not adequate by the designers of electronic circuits and equipment, since the mass or the thickness of the "core" is pertinent in assessing the gauge (from the electrical viewpoint) of the foil, while the "tooth" is not.
Thus, it is now believed that the foil is better characterized by its thickness, measured by micrometer, since it takes into account the profile (cross-section) of the foil and the ratio between thickness of the core and the matte height, or the "tooth" (combined matte height of the foil and the treatment).
Since micrometer measurement includes peaks of the base foil and the peaks of the treatment upon them, a foil with a pronounced matte side of the base foil and with a large amount of treatment will be thicker than a foil with a less pronounced base foil matte structure and a lesser amount of the treatment, even if the weight areas of both foils are the same. The thickness of 1 oz. foil can be as different as 1.8 mil and 1.4 mil. The industry trend is toward "thinner" foils in these terms. Such foils are referred to as "low profile". Foil with a rectangular cross-section would be considered ideal, theoretically, if the foil's bonding ability were not an important consideration. However, it is widely agreed that the matte height of the foil should not exceed 15% of the total thickness of the foil. Only such foils are desirable for use in fabrication of multi-layer boards, the most advanced and fastest growing segment of printed circuits industry.
Matte height is routinely measured by copper foil manufacturers and users, with a stylus type instrument which measures peak to valley amplitude. The trend toward the use of low profile foil in electronic applications represents a very serious dilemma to the foil manufacturers, since the users of copper foil are unwilling to lower their requirements in regard to peel-strength (bondability) of the foil, while insisting on the necessity of lower profile foil.
It is self evident that both the micro-roughness of the base foil (Rz of peak and valleys) and the bonding treatment contribute toward the profile (roughness) of the finished foil.
Traditionally, it has been understood that the rough matte side of the base foil (high Rz) followed with the high amount of the bonding treatment delivers the highest possible peel-strength, while the foil made in such manner is also characterized by high roughness (high profile, high Rz).
Clearly, the two of the printed circuit industry's most important requirements in regard to the characteristics of copper foil: good peel-strength, and low profile, are in conflict in terms of traditional foil fabrication concepts and practice. Therefore, it would be highly desirable to have commercially available an economical process for producing high quality copper foil having both a very low profile surface and a high peel strength.
A primary object of the present invention is to provide an improved process for high quality copper foil having a very low profile treated surface which has a high bond strength when laminated to a polymeric substrate, as well as a copper foil produced by such process and a copper clad laminate made with such foil. Other objects may become apparent from the following description of the invention and from the practice of the invention.